Several different types of currently available instruments may be used to measure the thickness and optical constants of a static thin film. Other systems are designed to dynamically monitor film thickness changes. Photoresist users, manufacturers, and raw material suppliers utilize both types of systems. In order to optimize raw materials and formulations they measure fundamental properties of the materials and simulate lithographic performance. In addition to the thickness and optical properties of thin polymer films, they need to measure the dissolution rates of the films in photoresist developers. The commercially available systems do not provide a cost effective solution to the needs of the photoresist designers and users. There exists a need for an inexpensive and reliable system that will perform both the static and dynamic functions in an expeditious and efficient manner.
Custom, noncommercial systems to measure dissolution rates of organic films have been used for years for testing photoresists and constituent polymers. The earliest systems were usually similar to the one shown in FIG. 1. A 632.8 nm HeNe laser 101 is used as a source of monochromatic radiation. The laser light is conducted by means of a first set of optical fibers in a bifurcated fiber optic cable 102 to the surface of a substrate 103 coated with a thin film of photoresist or polymer. The substrate is typically a silicon wafer as used in the manufacture of semiconductor devices. A fixture 104 is provided to maintain a near-normal angle of incidence between the substrate and the fiber optic cable while both are immersed in a container of developer 105. Reflected light is collected by a second set of fibers in the bifurcated cable and directed to an assembly 106 containing an optical filter and photodiode detector. The light emission from the fiber optic cable is not in the form of a narrow beam as originally emitted from the laser and, therefore, illuminates a relatively large area on the substrate. Given this broad area of illumination and the specular nature of the substrate, an image of the emitting fibers is reflected onto the collecting fibers without a need for critical alignment. In order to avoid interference from ambient light, a 632.8 nm interference filter may be included in the optical path before the detector. The photodiode signal is sent to an amplifier 107 and a strip chart recorder 108. This system is capable of collecting reflectance data from a single point on a substrate at a single wavelength.
In order to use the system depicted in FIG. 1, a coated wafer is mounted in the fixture with the end of the fiber optic cable in close proximity to the wafer. The mounting procedure is done while the fixture is removed from the developer. After opening the laser shutter and starting the strip chart recorder, the fixture is immersed into the developer. The recorded signal 109, which is modulated by thin film interference effects as the film thickness decreases during development, is referred to as an “interferogram”.
The analysis of the interferogram is usually done in a very simple manner. A sharp change in intensity occurs at the time when the sample is plunged into the developer; this marks the time when development starts. A development time for each peak may then be read from the chart. The film thickness at each minimum and maximum in the interferogram can be determined by counting peaks back from the end of development. The end of development may be identified because the modulation of the signal ceases. The thickness associated with each peak is then calculated as follows.                               t          =                                    N              ⁢                                                           ⁢              λ                                      4              ⁢              n                                      ,                            (        1        )                            where: t=film thickness,        N=number of half cycles in peak count,        λ=wavelength, and        n=refractive index of the film.        
The film thickness versus development time data then yields the desired development rate information.
In an improved version of the system shown in FIG. 1, the strip chart recorder is replaced with an automated data acquisition system. Automated peak finding and a simple method for interpolating between peaks are usually provided. Rigorous modeling of the reflectance is not usually incorporated into these systems because it is difficult to determine the incident intensity and to control other parameters.
A major disadvantage of the above system is the large quantity of developer, typically 500 ml to 2 liters, required to obtain a single set of reflectance data during development. A second disadvantage is that only one set of reflectance data can be obtained from each coated wafer. An additional disadvantage is that since two points are required to define the line, it is not possible to measure thickness below 200 nm for typical values of n and λ.
A more sophisticated prior art instrument utilizes a spectrometer to provide spectral reflectance during development. The additional data obtained provides more options for data analysis. The multi-wavelength system is also capable of measuring the thickness of a static film, but current systems have not been optimized to perform both static and dynamic functions. Also, the problems of inefficient utilization of coated substrates and developer remain.
A major improvement in capability was provided by the Perkin Elmer Development Rate Monitor (PE DRM), a commercial instrument designed to measure dissolution rates. The PE DRM simultaneously acquired data from multiple channels. The multi-channel capability is useful for simultaneously measuring dissolution rates of photoresist at different exposure dose levels. A resist-coated wafer is exposed with an array of different doses and thickness versus time data is collected during development for each zone of the array. The physical design of the PE system is very different from the simple single channel systems. The PE system utilizes a white light source to illuminate the exposure array on the wafer. The exposure array is then imaged onto a detector array. At some point in the optical system, a filter limits the radiation to quasi-monochromatic light. Large amounts of data can be collected and analyzed to provide dissolution rates corresponding to the given exposure doses. The PE multi-channel DRMs have been very popular for characterizing photoresists but are expensive. They also require large amounts of developer for each wafer and are not designed to make static measurements of film properties.